1. Field of the Invention
The present invention relates to a light guiding device for an elongate light source, especially a fluorescent tubular lamp, with a substantially tubular or tube sector body of transparent material for substantially coaxially receiving the light source and which at its internal surface is provided with grooves, notches or the like disposed parallel to the axis and engaging each other and having flanks.
2. Prior Art
A device of the kind mentioned in the introduction is known from Swiss patent 389 538. In the known device the grooves serve to disperse or fan out the distribution of light in a direction normal to the axis in order to reduce the light density immediately below the light source. The broad dispersion effect is attained by using pointed grooves with an vertical angle of about 60xc2x0. The question of light suppression relative to the direction of the axis of the light source is not addressed in the publication.
In elongate light sources such as fluorescent tubular lamps, distribution of light is often desired in practical applications which maintains a predetermined radiation angle outside of which light emitted from the light source is largely suppressed or faded out. Refractive or reflective devices may be used for such purpose. A known kind of reflective device is a reflective raster consisting of longitudinal and transverse reflector vanes. Combinations of refractive and reflective devices are also possible.
It is an object of the invention to provide a refractive light guiding
device simplifying the construction of lamps and, more particularly, an alternative to reflective raster lamps which is more cost-efficient.
The object is accomplished by a device of the kind mentioned in the introduction which in accordance with the invention is characterized by the flanks of the grooves seen in section normal to the axis being inclined at an angle of 40xc2x0 to 46xc2x0, preferably 43xc2x0, relative to the radial direction.
This results in a new improved kind of prismatic structure of a profile which is uniform throughout the length of the body. The function of the structure will be described in greater detail infra. The effect of the structure is a deflection of light rays closer to the radial direction. The light emission of a fluorescent tubular lamp enveloped (at least partially) by such a device is suppressed in an angular cone in the axial direction, i.e. the angle of emission normal to the axis of the fluorescent tubular lamp is restricted (for instance in a downward direction). As a result the usual vanes extending normal to the axis of reflective raster lamps are no longer required since the suppression of light in the longitudinal direction is taken over by the light guiding device in accordance with the invention.
Preferably, the grooves are distributed evenly over the internal circumference of the body, and in a particularly preferred embodiment there will be at least twenty grooves. In this manner the distribution of light over the range of emission is rendered more uniform.
In a preferred embodiment of the invention the flanks of the grooves are concave when seen in section normal to the axis. Scattering resulting from the structure of the grooves is thus minimized which leads to a sharper definition of the light suppression, and the degree of total light transmission (=efficiency) is maximized. Preferably, the radius of the flanks equals from half to twice the spacing between half the depth of the grooves and the axis of the body.
Preferably, the spacing between half the depth of the grooves and the axis of the body is between 1.4 to 1.7 times the radius of the light source to be received. This results in a particularly compact structure.
In any case, a refractive index of at least 1.45 of the transparent material is particularly advantageous since it causes the angle of suppression to be increased and the angle of emission to be decreased.
A further aspect of the invention relates to providing a lamp with an elongate light source, in particular a fluorescent light tube surmounted by a roof-like reflector and which is provided with a light guiding device of tubular structure enveloping the light source. In case a light guiding device structured as a tube sector is used it will surround the light source at its side opposite the reflector.